1. Field of the Invention
This invention relates to the enzymatic esterification of racemic carboxylic acids and racemic alcohols, and to the transesterification of alcohols, wherein resolution of the racemic acids and alcohols is provided.
Optical isomers (or enantiomers) are molecules that are mirror images one of the other and cannot be superimposed. They exist because of the presence of asymmetry in their structure, typically involving an asymmetric carbon atom.
Their physico-chemical properties are identical except for in the direction of rotation of the plane of polarized light and their interaction with other asymmetric molecules. This renders the separation of optical isomers difficult.
Optical purity, the presence of only one optical isomer, of chemicals used as drugs is important because the molecules with which drugs interact--receptors, enzymes, etc.--are asymmetric and therefore may behave differently with the two isomers. In some cases, the non-active isomer is responsible for important side effects.
2. Description of the Prior Art
The separation of optical isomers can be achieved by the classical precipitation of diastereoisomers (Willen, S. H., Collet, A. and Jacques, J., Tetrahedron, 33, 2725-2736, (1977), or by kinetic resolution, using chemical reaction (Katsuki, T. and Sharpless. K. B., J. Am. Chem. Soc., 102 5974-5976 (1980), chemical (Martin, V. S., Woodard, S. S., Katsuki, T., Yamada, Y., Ikeda, M. and Sharpless K. B., J. Am. Chem. Soc., 103, 6237-6240 (1981) or enzymatic catalysis. Margolin, A. L., Enzyme Microbial Technol. 15, 266-280 (1993).
Enzymatic resolution is based on the difference in the rates of the reactions catalyzed by an enzyme on the two isomers of the molecule of interest. It can be performed either through hydrolysis, synthesis, oxidation or group transfer, depending on the functions present on the molecule.
In the case of racemic acids and alcohols, the esterase catalyzed stereoselective hydrolysis of esters has been extensively described (in the example shown in scheme I below, the acid moiety of the ester bears the asymmetric centre and the (R)-acid and (S)-ester can be separated by classical methods). ##STR1##
The esterification or transesterification in organic or aqueous/organic media has also been described. In scheme II below, the (S)-acid is esterified to produce the (S)-ester and leave (R)-acid untouched. In reaction scheme IIA below, if the alcohol bears the asymmetric centre and the (S)-form of the alcohol reacts in the esterification, the (R)-form remains untouched and the (S)-ester is produced.
In scheme III, the alcohol bears the asymmetric centre and the (S)-form of the alcohol reacts in the transesterification while the (R)-form remains untouched. ##STR2##
In the above reaction schemes II, IIA and III R is an alkyl group containing 8 or more carbon atoms for acids, and containing 10 or more carbon atoms for alcohols and esters, (i.e. long enough so that the vapour pressure of the alcohol, acid or ester is low at the temperature of the reaction and the alcohol does not evaporate). R is an alkyl group which may be saturated or unsaturated, linear or branched, substituted or unsubstituted, an aryl group which may be substituted or unsubstituted, heterocyclic, substituted or unsubstituted, or any other group that does not contain a chemical function that will react with the alcohol ROH or acid RCOOH, and R" is a lower alkyl group containing 1 to 4 carbon atoms (to allow the easy evaporation of the by-product alcohol, R" OH formed during the transesterification reaction).
The following limitations or drawbacks of present apparatus, product or process, are noted.
For acids and alcohols, aqueous hydrolysis is limited to esters that are soluble in water. Aqueous/organic two phase systems can be used to overcome this limitation at the expense of increased complexity.
Esterification and transesterification in organic solvents or two phase aqueous/organic systems have to overcome yield limitation caused by the by-product of the reaction, namely, water or short-chain alcohol (R.sup.11 OH).